Low-power frequency modulation (FM) transmission in the FM broadcast channels is a permissible application in many countries for personal use as long as the transmitted power is in compliance with local regulatory standards. On the other hand, cellphones have been widely used almost in all regions around the world. In U.S. patent application Ser. No. 12/137,535, a system and method are disclosed to allow a transmitter to automatically select a clear radio channel that is not occupied by others so as to achieve the best possible transmission quality. In U.S. patent application Ser. No. 12/495,796, a system and method are disclosed which allow an audio link automatically established between an audio transmitter and an audio receiver. The system and method disclosed in U.S. patent application Ser. No. 12/495,796 rely on the clear channel technique disclosed in U.S. patent application Ser. No. 12/137,535 and a handshaking technique to establish the audio link automatically. Wireless earphones are one such application based on the invention of U.S. patent application Ser. No. 12/495,796, where the transmitter may be coupled to a cellphone or a portable media player to allow the audio heard on a matched wireless earphone.
The wireless earphone application can be further extended to allow two-way audio communication between two devices. In other words, the earphone becomes a speakerphone or a hand-free headset which not only receives audio but also provides audio using a microphone. The speakerphone comprises a speaker to deliver amplified audio transmitted from a portable device such as a cellphone or multimedia player, and a microphone to pick up the sound at the remote side and to relay it to the portable device. A speakerphone according to the above description will be useful for group conference call. For applications in a large group conference call, the remote speakerphone may use multiple microphones to pick up sound from multiple conference participants. The wireless speakerphone is also an ideal application for hand-free cellphone operation during driving a vehicle, where the wireless speakerphone can be mounted on the visor or a place close to the mouth of the driver and the wireless speakerphone communicates with the cellphone through the wireless audio link. Therefore, the driver can have conversation through the cellphone without holding the cellphone close to his ear/mouth. The wireless speakerphone can be based on the low-power FM technology which is mature and readily available worldwide. Such duplex audio link will be very useful and desirable for various wireless audio applications. However, there are broadcasters using assigned channels which are not suitable to use these channels for low-power transmission for personal use. Also, there may be channels being used by other nearby which may cause interference if such channels are used. Furthermore, it is desirable to have an easy to use speakerphone system that can automatically coordinate channel selection and tuning.